Ethylene oxide (ETO) is widely used in the chemical industry as a feed stock chemical, and is also commonly used as a sterilant gas, in for example, hospitals and medical equipment suppliers. ETO is one of the most efficient sterilant gases available, especially for those articles that cannot be sterilized by exposure to high temperatures. (A. R. Reiche “Safe Use of Ethylene Oxide in the Hospital Environment”, in “Essentials of Modem Hospital Safety” Ed. W. Charney, J. Schirmer, Lewis Publishers, Chelsea Mich. (1991)). Ethylene oxide is a highly toxic gas, and the OSHA permissible work-place exposure limit for ethylene oxide is only 1.0 ppm averaged over an eight-hour shift (29 CFR 1910.1047). Thus, it is important to have adequate protection of personnel who are using ethylene oxide and it is common practice to use gas detection monitors to warn of potential leaks or other exposure to ethylene oxide. In a typical hospital application, gas detection monitors will be located near the sterilizers and in storage areas where ethylene oxide gas is kept. In addition, ethylene oxide monitors may also be deployed near or within abators used to destroy any ethylene oxide remaining in the waste gas stream from the sterilizers.
Currently, the two most frequently used technologies for continuously monitoring ETO in the workplace are electrochemical gas sensors and gas chromatography (GC). While GC has very good sensitivity and reproducibility, it has several drawbacks. Specifically, the GC method is inherently a spot check method since a sample is drawn into the instrument and then analyzed. For a typical monitoring application, at least four sampling points need to be analyzed, drawing gas from various locations. These points are sampled sequentially, not simultaneously, and with cycle times between samples on the order of two to three minutes there is every possibility that a leak of ETO could potentially not be detected for up to twelve minutes. In addition, GC based instruments tend to be large and bulky, require a supply of carrier gas and are usually expensive. Despite these limitations, GC based instruments are still widely used for monitoring ethylene oxide.
The other major technology for detecting ethylene oxide employs electrochemical sensors. The ethylene oxide enters the sensor, usually by diffusion from the ambient air, but sample draw equipment may also be used to deliver the ethylene oxide to the sensor. Once the ethylene oxide is in the sensor, it is oxidized at the working electrode and the resulting electric current is measured to provide a quantitative indication of the ethylene oxide concentration. Instruments using ETO electrochemical sensors are available from companies such as Chemdaq Inc., Pittsburgh Pa.
While instruments employing electrochemical sensors provide continuous monitoring, alarm upon high exposure to ETO, and have the sensitivity to detect 0.1 ppm ETO, they also possess drawbacks. In particular, the electrochemical sensors that are used to detect ETO respond to a wide range of easily oxidizable compounds, such as alcohols and carbon monoxide. Alcohols in particular are a problem because of their ubiquitous use in hospital environments. This cross sensitivity can result in false alarms and a loss of user confidence in the instruments.
Alcohols are easily oxidized, primary alcohols such as ethanol (CH3CH2OH) are oxidized to the corresponding carboxylic acid (e.g. ethanol to acetic acid) and secondary alcohols are oxidized to the ketone (e.g. isopropanol to acetone). If a monitor is exposed to either ethylene oxide or an alcohol vapor the monitor will give a response and at higher concentrations the monitor will give an alarm. However, the user does not know whether the monitor is responding to ethylene oxide and so should take remedial action or if the monitor is responding to the alcohol in which case it is safe to ignore the alarm. Consequently cross sensitivity of alcohols is one of the major drawbacks of using electrochemical sensors for ethylene oxide.
Alcohol is typically found in many medical environments, since it is a standard method for sterilizing equipment, work surfaces and skin. Alcohol is a common additive in many products, for example the hand sterilizers that are commonly used in medical facilities contain a gel comprised of about 80% w/w ethanol. For the purposes of this disclosure, the term alcohol corresponds to any small aliphatic alcohol, and especially ethanol and isopropanol.
Alcohol vapor is of particular concern in those locations where both alcohol and ethylene oxide may be found. For example a sterilization room in a hospital may use an alcohol based cleaner/sterilizer to sanitize the work surface (counter tops etc.) near the sterilizers. Since most alcohols are volatile (B.P. ethanol=76° C.), alcohol vapors are often found in the same vicinity as ethylene oxide monitors.
Several gas sensors incorporate chemical filters in an attempt to reduce the cross sensitivity to interferent gases. Filters are placed in the gas path such that all the gas that reaches the sensor must pass through the filter. The filter is designed to remove the interferent gas but not the target gas. However chemical filters cannot be used for all types of gas sensors being limited by the available chemistry to differentiate between the target and interferent gases. The use of chemical filters within sensors is well known in the prior art; for example, Tantram and Chan in U.S. Pat. No. 4,633,704 describe the use of a soda lime filter to prevent hydrogen sulfide from giving a response on a carbon monoxide sensor; and Warburton and Sawtelle described a filter based on silver salts also to remove hydrogen sulfide in U.S. Pat. No. 6,284,545. There are many filter materials that will remove alcohols from a gas stream, for example, charcoal filters are commonly used to protect carbon monoxide gas sensors from interferent gases including alcohols, as is illustrated by Kiesele et al in U.S. Pat. No. 5,865,973 and by Martell et al in U.S. Pat. No. 5,744,697, but no filters are known that remove easily oxidizable gases but also allow ethylene oxide to pass through substantially unimpeded.
One common type of filter employs an oxidizing agent, such as potassium permanganate. Commercially available potassium permanganate impregnated filter media for air filtration are available from companies such a Purafil Inc., Doraville, Ga. Potassium permanganate is a strong oxidizing agent, and oxidizes any alcohol which comes into contact with the filter medium, thus preventing it from entering the sensor.
The drawback is that ethylene oxide was also found to be oxidized by potassium permanganate and so the filter will prevent the ethylene oxide from getting into the sensor as well. Thus this type of filter is only suitable for relatively few types of gas sensors, such as those for carbon monoxide. Similarly ethylene oxide readily absorbs on common adsorbents such as activated charcoal and so these filters also cannot be used with ethylene oxide sensors. No chemical filter is available in the prior art that will allow ethylene oxide to pass through, but which will remove interferences such as alcohols and carbon monoxide.
Ethylene oxide is considered a highly reactive gas (hence its great toxicity), whereas alcohols are typically much less reactive, and considerably less toxic, as illustrated by the number of ethanol containing beverages currently available. In addition to being more reactive than ethanol, ethylene oxide reacts with acids and bases via hydrolysis to produce ethylene glycol (HO—CH2CH2—OH), which is chemically and physically similar to ethanol (CH3CH2—OH) making chemical differentiation more difficult.
At present the electrochemical sensors offer the best all round technology for detecting ethylene oxide, despite their cross sensitivity drawbacks. Clearly, there is a need for a filter that is suitable for use with an electrochemical gas sensor which will prevent interference from alcohols and other easily oxidizable volatile chemicals but which will still allow ethylene oxide to reach the gas sensor.